1. Technical Field
The subject matter described herein relates to pressure sensors.
2. Background
Micromechanical devices commonly are used to create many types of sensors, including but not limited to pressure sensors, accelerometers, gyroscopes, and magnetometers. As time progresses, customers continue to demand a reduction in the size, cost, and current consumption of such sensors through a consolidation of the sensors into combination sensors. However, different fabrication processes often are used to fabricate different types of sensors. Using a different fabrication process for each type of sensor complicates the integration.
Conventional pressure sensors typically are formed in electronic packages that have a membrane (a.k.a. diaphragm) extended over a cavity in a substrate, such that the membrane is coplanar with the substrate. A relative change of a pressure above the membrane with respect to a pressure below the membrane causes a net force that deforms the membrane. Capacitance-based principles can be used to detect a magnitude of the change, such that a greater capacitance corresponds to a greater magnitude.
For example, strain sensors may be incorporated into the membrane. The strain sensors may include a piezoelectric material that is formed a silicon substrate of the pressure sensor from which the membrane may be made. In another example, an electrode may be placed in the cavity, and as the membrane moves closer to the electrode due to deformation of the membrane, the capacitance increases. In accordance with this example, when a voltage is applied between the membrane and the electrode, a difference between the charges on the membrane and the electrode is related to their separation.
In such conventional pressure sensors, supports for the membrane are attached to the surrounding electronic package. When the electronic package is attached to a circuit board, temperature and stress changes can be transmitted into the supports for the membrane and thereby create a false reading of the magnitude of the change. Moreover, it is well-known that piezoelectric materials are relatively sensitive to changes of temperature.
Calibration techniques sometimes may be used to remove inaccuracies of the reading that are based on the temperature and stress changes if the temperature and stress changes are due to tensions internal to the sensor, such as those arising from differing coefficient of thermal expansion (CTE) values among materials internal to the sensor. For instance, a reference sensing element may be used in combination with a primary sensing element so that a differential reading between the primary and reference sensing elements may be made. However, using such calibration techniques may consume a substantial amount of area on the substrate, increase cost of the sensor, and/or not adequately remove the inaccuracies. Furthermore, compensation for stress changes that are not internal to the sensor may not be possible.